When it comes to the mysteries of physics there aren’t many things more mysterious than the so called dark energy. It is a prime suspect when it comes to the accelerated expansion of the universe. Even though it is an invisible energy, it is all around us. Dark energy is distributed all among the universe with a low density of 10-26 kg/m3 and affects no only the expansion of the universe but also the formation and evolution of the clusters of galaxies.
Two proposed forms for dark energy are the cosmological constant (a constant energy density filling space homogeneously) and scalar fields which are basically dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant, however Scalar fields change extremely slow, thus are hard to distinguish from the cosmological constant.
The big problem with the dark energy is that its nature is unclear. And although there is a variety of theories that try to explain it, none succeed giving the correct numbers. A major outstanding problem is that most quantum field theories predict a huge cosmological constant from the energy of the quantum vacuum, more than 100 orders of magnitude too large.
Stuff that makes up the universe
Now physics wouldn’t be physics if there wouldn’t be a bunch of alternative theories to explain the same phenomenon, some obscure while others more popular. So let’s look at some of them.
The first alternative is a suggestion by Christos Tsagas from the Aristotle University of Thessaloniki in Greece, who believes that the accelerated expansion of the universe is only an illusion caused by the fact (he calls it the “Dark flow“) that our section of the universe, roughly 2.5 billion light years wide, is moving at a different speed and direction than other sections. If ‘dark flow’ is real, it may provide a simpler understanding of the cosmos than current theories. However, many experts are still sceptical as more observations are needed.
Another alternative, which seems to be getting more attention lately is that Einstein’s general theory of relativity is somehow flawed or incomplete. Some physicists believe that it’s the failure of general relativity on very large scales. Unfortunately the calculations always seem to return results that are not compatible with the observations.
The other type of alternatives come from from string theory, brane cosmology and the holographic principle, all of which have not yet been proved. As Leonard Susskind says, in string theory it’s possible and even desirable that our universe is only one of many universes in the so called multi verse. Then, if it is true, every universe could have a different magnitude of the vacuum energy, which could account for the dark energy in our universe. Of course, there are many critics as none of this has been proven yet and, let’s be honest, the idea of 10500 other universes that exist besides our one is kind of hard to wrap your mind around isn’t it? And the whole subject of detecting other universes is also highly speculative at this point.
Other interesting idea is the double metric tensor for space-time. It has been argued that time reversed solutions in general relativity require such double metric for consistency, and that both dark matter and dark energy can be understood in terms of such solutions.
Last but not least, a new idea called Dark Magnetism. It is basically a modification of the Maxwell’s laws on the large scale, which would explain the acceleration of the universe. This one’s a little tricky so check out the original research paper here. Also it was featured as the main article in last weeks New Scientist so be sure to check it out also.
In conclusion, a single explanation that would give out correct numbers doesn’t exist as of today, but there are many promising theories and a lot of work done to test them. Only time will tell, what really is accelerating our vast universe.